1. Field of the Invention
This invention relates to a system for withdrawing permeate from a substrate through a filter. More particularly, this invention relates to a system adapted for withdrawing permeate from a substrate through a filter and for cleaning the filter in situ. A method is also provided.
2. Related Art
Filtration systems provide barriers in order to allow permeate to be drawn off from a substrate through the filter while concentrate is left behind. For example, filtration systems have been used as barriers to retain biosolids in biological reactors. In such filtration systems, membranes have been proposed as the barrier. Such membranes can be provided in the form of hollow fibers, tubes, or rolls, for example.
For the purpose of illustration, leachate treatment systems for wastewater treatment applications may use a membrane separator in order to separate feed into permeate and bio-mass. Such systems are available, for example, under the trademark ZEEWEED from Zenon Environmental Inc. of Ontario, Canada. The ZEEWEED system uses a submersible membrane cassette to bring about bio-oxidation to oxidize organic matter in the feed. Membranes are used to retain bacteria in the system for essentially complete oxidation and to provide high effluent quality.
It has been recognized that it is important to keep membranes used in such systems "clean" because, after some period of use, a fouling film or "bio-film" can form on the membrane, thereby reducing the flow of permeate through the membrane. A buildup, whether organic or inorganic, may form on the membrane's outer surface, inner surface, and/or in the membrane's pores that extend through the membrane's wall. Such a buildup on the membrane has, therefore, been recognized to decrease the performance of the membrane as a viable filter.
U.S. Pat. No. 5,403,479, issued to Smith et al. ("In Situ Cleaning System for Fouled Membranes") provides ample background as to the nature and extent of the fouling problem that tends to plague the bio-filtration industry. U.S. Pat. No. 5,403,479 is incorporated herein by reference in its entirety. As one possible solution to the problem of membrane fouling, Smith et al. proposed in the '479 patent a cleaning system for substantially restoring transmembrane flux in fouled, porous/semipermeable microfiltration or ultrafiltration membranes used to recover purified water from contaminated or "dirty" water. Specifically, Smith et al. proposed cleaning a module containing a membrane, without draining feed from the module, by introducing a chosen cleaning fluid into the permeate and recycling it through the lumens of hollow fiber membranes at low pressure not exceeding the bubble point of the fiber. The process proposed by Smith et al. in the '479 patent cleans from the permeate side of the membrane; that is, through the lumens of the hollow fibers.
U.S. Pat. No. 5,248,424, issued to Cote et al. ("Frameless Array of Hollow Fiber Membranes and Method of Maintaining Clean Fiber Surfaces While Filtering a Substrate to Withdraw a Permeate") proposed another approach for maintaining the performance of filtration membranes; more specifically, a frameless array of hollow fibers. Cote et al. proposed in the '424 patent a system to reduce the build-up of growing microbes or the deposits of inanimate particles upon the surfaces of fibers kept awash in bubbles of a fiber-cleansing gas ("scrubbing gas"), particularly an oxygen-containing gas ("air-scrubbed"). The build-up is essentially naked when the fibers are buoyantly freely swayable in a frameless array submerged in a substrate through which the bubbles rise with sufficient physical force of impact to keep the fibers essentially free of deleterious deposits. Similar solutions were proposed by Mahendran et al. in U.S. Pat. No. 5,639,373 ("Vertical Skein of Hollow Fiber Membranes and Method of Maintaining Clean Fiber Surfaces While Filtering a Substrate to Withdraw a Permeate") and by Henshaw et al. in U.S. Pat. No. 5,783,083 ("Vertical Cylindrical Skein of Hollow Fiber Membranes and Method of Maintaining Clean Fiber Surfaces").
In International Publication No. WO 98/37950 ("Portable Reverse Osmosis Unit for Producing Drinking Water"), Daly et al. proposed a method and apparatus for producing drinking water from impure water wherein hollow tubular membranes of the system are periodically back flushed with retentate by directing the retentate to the inside surfaces of the membranes and by passing the retentate through the membranes, thereby dislodging particles from the outside surfaces. When chemical cleaning of the membranes is required in the method and apparatus proposed in the '950 publication, cleaning solution is pumped from a tank into the membranes.
In Australian Patent Application No. AU 9676300 (corresponding to International Publication No. WO 97/18887), Cote et al. described a method for cleaning immersed membranes in situ, wherein effluent contained in the tank is at least partially emptied in order to expose the membranes to the air, and cleaning solutions are passed through the pores of the membranes along a flow path opposite to the filtration flow of the effluent by delivering cleaning solution to the permeate side of the membranes. A shut-off valve is opened in order to drain off effluent from a treatment tank. Cleaning solution is then introduced into the membranes from a reservoir. In another embodiment, four tanks are supplied with effluent. When one wishes to clean the membranes in one of the tanks, the contents of the selected tank are transferred into the other tanks. Cleaning solutions are fed into the membranes of the empty, selected tank from reservoirs.
Although significant effort has been expended to resolve this recognized problem of fouling, improvements regarding the "cleaning" of filtration systems such as those that employ membranes are still in demand, whether the membranes are provided in the form of hollow fibers, tubes, rolls, or other membrane configurations. Specifically, despite these significant advances in the art of filter cleaning, and despite the purported ability of such proposed systems to prolong the throughput rate of the membranes used as filters, it has been discovered that, in some instances, the membranes must eventually be removed from the process for a thorough cleaning such as a deep chemical cleaning. The need to remove a filter from a system such as a biological reactor is of course time consuming, expensive, labor intensive, and generally undesirable. Moreover, it often requires that the system be at least partially shut down during the cleaning process while the filter is removed.
For example, it is undesirable to remove a submersible membrane unit from a biological reactor and to move the submersible membrane unit to a separate tank for cleaning. Membrane assemblies can be quite large and quite heavy. Also, in the case of an industrial biological reaction system, the biological reactor vessels in which membrane assemblies are used can be quite tall, thereby requiring expensive and cumbersome rigging equipment for removal. Furthermore, the various "plumbing" connections to such membrane assemblies must be disconnected and subsequently reconnected in order to bring about membrane assembly removal and replacement, respectively. It will also be understood that over-head clearance may not be available for removing such membrane assemblies easily, and when such systems are removed, the process of doing so can create quite a mess. Also, external tanks dedicated to separate cleaning operations for off-line cleaning procedures require significant floor or ground space and numerous "plumbing" connections.
Accordingly, the need remains for an improved system for withdrawing permeate from a substrate through a filter and for cleaning the filter in situ. A corresponding method is also needed.